Minerals

Background:

Students' interest in rocks can be used to introduce the elements in minerals and the classification schemes used by geologists. This lesson introduces atoms, elements, minerals, rocks, and the chemical composition of Hawaiian rocks. The depth to which you dive into this material depends on the grade level you teach. For example, a "show-and-tell" of minerals would be appropriate for K-3 students, whereas students in grades 4-6 could explore the physical characteristics of minerals. Classification and identification of rocks would be appropriate for grades 7-8. High school students could investigate the chemistry of minerals and rocks.

An atom is the smallest subdivision of matter. An atom consists of three types of particles. The nucleus, or center, of an atom contains neutral particles, called neutrons, and positively charged particles, called protons. Around the nucleus is a thinly populated region that contains electrons, particles with a negative electrical charge. You can picture an atom as a miniature solar system. The sun represents the protons and neutrons. Electrons orbit the nucleus much like planets orbit the sun (except that planets remain in the same plane). In an atom, the number of protons and the number of electrons are equal; therefore, the atom is neutral or has no electrical charge. If an atom gains or loses electrons, the charge of the atom becomes negative or positive, respectively. Charged atoms are called ions. If you have ever shuffled your feet on a carpet and then touched a door knob, you are familiar with an electrical charge. An element is a substance which cannot decompose into other substances by ordinary chemical means. One atom of an element will have all the characteristics of that element. Although you may not realize it, you are already familiar with several elements. For example, the element carbon is in diamonds, used for jewelry, and in graphite, used in pencil lead. We also use the elements gold and silver for jewelry. We need the element oxygen in the air we breathe. We worry about the carbon dioxide emitted by our cars. Carbon dioxide is a compound, or a combination of two or more elements. In the case of carbon dioxide, two atoms of oxygen combine with one atom of carbon. Another common compound, table salt, is made of the elements sodium and chlorine.

Table 1. Average Composition of the Crust

Element (Symbol)	Weight percent
__________________________________

Oxygen (O)		46.6
Silicon (Si)		27.7
Aluminum (Al)	  	 8.1
Iron (Fe)		 5.0
Calcium (Ca)		 3.6
Sodium (Na)		 2.8
Potassium (K)	  	 2.6
Magnesium (Mg)	  	 2.1
			98.5
__________________________________
Eight elements combine to make most minerals and rocks found in the Earth's crust. The elements are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. The letters in the parentheses are abbreviations. You may already be somewhat familiar with some forms of these elements. Silicon combines with two atoms of oxygen (in the nomenclature of chemist, SiO2,or silica) to make the mineral quartz, which is common on many mainland beaches. Silica is also used for window glass. Aluminum is used for soda cans. Iron is used for the frame of automobiles and reacts with oxygen in the air to produce rust. Magnesium is used in flares. Calcium is a major component in our teeth and bones. Sodium is in table salt. Potassium is used in fertilizer. Titanium is combined with other metals to produce alloys, which are stronger metals.

Plagioclase is a common mineral in many volcanic rocks. Photo by Steve Mattox.

A mineral, by definition, must satisfy five conditions:

  1. It must be naturally occurring.
  2. It must be inorganic.
  3. It must be a solid element or compound.
  4. It must have a definite composition.
  5. It must have a regular internal crystal structure.
This definition excludes the thousands of compounds invented by humans in laboratories because these compounds are not naturally occurring. Compounds that are found in only plants or animals are also excluded. Liquids are excluded because they are not crystalline, their atoms are free to move. Minerals can be a single element, like diamond, which is made of carbon of compounds of two or more elements, like quartz, which contains one silicon and two oxygen atoms. Definite composition indicates that a chemical analysis of a given mineral will always produce the same ratio of elements. For example, quartz will always have one silicon for every two oxygen atoms. Therefore, minerals can be expressed by chemical formulas, such as SiO2 for quartz.

Common ions in minerals. Charges and relative sizes are shown.

In some minerals, elements of similar size and charge substitute for each other. The amount of iron and magnesium in olivine varies because these elements can substitute for each other in the structure of the mineral. The formula for olivine, (Mg,Fe)2SiO4, indicates that for every two magnesium and/or iron atoms, there are one silicon atom and four oxygen atoms. Substitution of sodium for calcium occurs in the mineral plagioclase.

Regular internal crystalline structure indicates that the atoms are arranged in a regular repeating pattern. This diagram shows the structure of the mineral halite. Atoms of chlorine and sodium are arranged in a three dimensional repeating pattern.

Another basic arrangement, the silica tetrahedron, consists of one silicon atom surrounded by four oxygen atoms at the corners of the tetrahedron. The silica tetrahedron is a basic building unit for a major group of minerals called the silicates. This diagram shows four representations of the silica tetrahedron. A. Oxygen is represented by the white spheres and silicon by the smaller red sphere. B. An expanded view with rods representing bonds between the atoms. C. Diagrammatic representation of the tetrahedron, with four points representing the locations of oxygen atoms. D. Diagrammatic representation of the tetrahedron looking down from above.


By sharing adjacent oxygen atoms, the tetrahedron can form chains, sheets, and three-dimensional frameworks. A. The structure of olivine is based on isolated silica tetrahedra. B. The pyroxene minerals are made of a single chain of tetrahedra. C. The amphibole minerals are made of a double chain of tetrahedra. D. Micas, like biotite, are sheets of tetrahedra. E. Framework silicates, like plagioclase and quartz, are three-dimensional networks of silica tetrahedra.

Table 2. Common minerals in volcanic rocks.

________________________________________________________________________

Light-colored minerals (formula)		Dark-colored minerals (formula)
________________________________________________________________________

Feldspar:				Olivine	  (Mg,Fe)2SiO4
	Plagioclase	(Ca,Na)AlSi3O8	Pyroxene:
	Orthoclase	KAlSiO8		    Hypersthene   Mg,Fe)SiO3
	Anorthoclase	(K,Na)AlSi3O8	    Pigeonite	  (Mg,Fe)SiO3
Quartz			SiO2		    Augite	  Ca(Mg,Fe)Si2O6(Al,Fe)2O3
Nepheline		NaAlSiO4	Biotite		  K(Mg,Fe)3AlSi3O10(OH)2
					Magnetite	  Fe3O4
					Hematite	  Fe2O3
________________________________________________________________________
Click here to see samples of some of these minerals.

More than four thousand minerals have been discovered. Fortunately for geologists, only about one hundred of these are abundant and common in the rocks of the Earth's crust. The list of common minerals in volcanic rocks is even smaller. The minerals can be grouped by color. Minerals that contain iron (Fe) and magnesium (Mg) are dark in color. These minerals are often referred to as mafic or ferromagnesian minerals. The absence of magnesium and iron in a mineral results in a relatively lighter color. Geologists use the physical properties of a mineral to identify it in the field. These properties include color, streak, luster, cleavage, specific gravity, and hardness. The physical properties of common minerals are reviewed by Macdonald and others (1983).

Other Lessons         To VolcanoWorld